Using Two Indicators of SPImod and SDImod for Comparative Assessment of Meteorological and Hydrological Droughts in Tajan Basin

Drought is a natural, repeatedly and temporary event, that is caused by decreasing in rainfall amount than in long term average whether it can happen in every climate, and is one of the disturbing events in nature that causes the most damage to water resources. In this study was used modified Indexes Standardized Precipitation (SPImod) and streamflow drought index (SDImod) for purposes of temporal assessment and analysis of droughts occurrence, monthly data of 15 rain gauge and 4 hydrometric stations in Tajan basin were used and indices in six timeframes short-term (one-month and three-month) intermediate term (six -month and nine -month periods) and long-term (12 months and 24 months were calculated in MATLAB software. In part of the research, Pearson correlation coefficient between precipitation and flow with different time delays was calculated to determine the delay of currents' delay to precipitation in real mode. Then the modified Streamflow drought index (SDImod) with modified Standardized Precipitation Indexe (SPImod) time series at various temporal interval using Pearson correlation was studied in the whole of basin and sub basins also compared separately. The results showed that temporal relationship between hydrological and meteorological drought occurrence is significant at 99 % level with together and this correlation is maximum in the 9 months' time step in Tajan basin. Also Drought occurrence in case study area showed its effect with a delay of 1 to 7 months on surface water resources.